This invention relates to biological permeable barriers for creating a xe2x80x9cbio-trenchxe2x80x9d or xe2x80x9cbio-curtainxe2x80x9d to clean contaminated groundwater. Specifically, the present invention relates to an apparatus and method to biodegrade contaminates in groundwater as the groundwater contacts and passes through the immobilized cells of the xe2x80x9cbio-trenchxe2x80x9d or xe2x80x9cbio-curtainxe2x80x9d during groundwater flow or movement.
Today""s release of the contaminants to the groundwater is increasing. With over 50% of the fresh water used in the United States coming from groundwater, contamination of this resource by xenobiotic chemicals represents a potential serious health and environmental problem. Toxicity, accumulation, and persistence of contaminants found in groundwater are just a few of the reasons for concerns.
Several methods of on-site aquifer restoration have been utilized recently to remove contaminates from groundwater. Chief among these methods are the pump and treat method in which the water is pumped out and treated, and the permeable barrier in which some type of filtering agent or reactive agent is placed in the ground to contact the contaminated water.
Conventional aquifer restoration alternatives such as pump and treat or on site remediation are not generally commercially effective for most forms of contamination. These technologies have numerous problems associated with them which include: management of large volumes of water, potential production of undesirable by-products from the reaction with the contaminate, production of waste sludge from the reaction of the filtering agent with the contaminate, the exhaustion of the filtering agent or reactive agent and need to replace it to continue treatment, undesirable effects on hydraulic characteristics in uncontaminated parts of the aquifer (change in direction of water movement), and the labor or energy intensive nature of the process.
An alternative to conventional groundwater treatment processes is the use of barriers which are permeable to water, but prevent the migration of contaminants. They are referred to as permeable barriers. In-situ permeable barriers are a relatively new cost-effective technology that can be used in groundwater remediation of shallow aquifers. Permeable barriers are installed as permanent, semi-permanent, or replaceable units across the flow path of a contaminant plume. Permeable barriers allow water to move passively through while precipitating, sorbing, or degrading the contaminants. These mechanically simple barriers may contain metal-based catalysts for degrading volatile organics, chelators for immobilizing metals, nutrients and oxygen for microorganisms to enhance bioremediation, or other agents. Degradation reactions may break down the contaminants in the plume into harmless byproducts. Crushed limestone, peat, and powdered activated carbon are also several effective barrier mediums that have been used to adsorb or precipitate contaminants.
Advantages of these barriers include the following: simple installation, simple recovery and replacement of the material, low operation maintenance, less surface disruption, less labor, and less energy are required than other remediation technologies; and comparatively quick installation and containment of contaminants.
One example of such non-biological permeable barrier is a mixture of powdered activated carbon (PAC) and sand. The PAC/sand mixture has been shown to be a successful medium for benzene removal in trench-based permeable barrier. The physical uptake of different mixtures (3% and 10%) of PAC/sand and nonabsorbent material such as sand and zeolite have been used. Another non-biological permeable barrier containing an iron-based catalyst has been used to reduce the concentration of trichloroethene (TCE) by 95% and the tetrachloroethene (PCE) concentration by 91%.
Rael evaluated possible permeable barrier media designed to remove benzene in-situ from ground water. Effectiveness of several common material including coal, powdered-activated carbon (PAC), peat, and zeolite were evaluated in a series of batch and column studies with an initial benzene concentration of 50 mg/L. Silica sand was used as an inert matrix and was mixed with PAC to produce either 3% (by weight) or 10% PAC/sand mixtures. Based on their results, a mixture of PAC and sand was considered the most successful candidate. However, these authors observed that when the barrier reached its treatment capacity it had to be replaced with fresh media. The barrier medium allowed the flow of contaminated water but adsorbed the contaminant preventing further migration. This technology is limited to the depth accessible by trenching equipment and therefore would be applicable in shallow aquifer systems of less than 30 m.
Morrison and Spangler have explored chemical barriers as a passive in-situ water-treatment system. Precipitation barriers (hydrated lime) and sorption barriers (ferric oxyhydroxide) for removing uranium from ground water were studied. Chemicals used in the barrier were placed in the subsurface either by lining a disposal site, by trench and fill, or by injection. Dissolved contaminants became part of the immobile solids of the aquifer, by either precipitation or adsorption, as the contaminated groundwater passed through the chemical barrier.
In 1991 Thomson et al. examined the concept of designing permeable barriers to remove groundwater contaminants in-situ. Permeable barriers constructed by trenching had two advantages: 1) accessibility of the medium placement and 2) ease of recovery of medium by re-excavation. Permeable barriers were classified as either passive or active. An active barrier required continuous operation and maintenance while a passive barrier required no operation or maintenance once the medium is in place. An example of active barrier, in-situ air stripper was investigated and compared with conventional packed tower air stripping. It was determined that: 1) the trench-based stripping needed high pressure air compressors, but no water pumping equipment was needed which made the operating cost less; and 2) biostimulation did occur from the oxygen, resulting in a combined air stripping and biodegradation of volatile organic contaminants.
The drawbacks of physical or chemical barriers that were mentioned above are production of waste sludge from the reaction of the filtering agent with the contaminants, the exhaustion of the filtering agent or reactive agent and need to replace it to continue treatment.
One known method to completely destroy the contaminants into the harmless by products in the water is biological degredation. Biological processes are carried out by bacterial species that are capable of using organic compounds as their carbon source. Because of numerous advantages of biological processes, bioremediation has emerged as a viable technology to use microorganisms as effective agents to remove organic compounds from groundwater. The most common approach for large-scale bioremediation has been to inject nutrients into the ground water to simulate contaminant-degrading organisms. This approach has not proven to be reliable due to biofouling the stimulated population and contaminants into contact.
Another approach, called bioagugmentation, involves the addition of bacteria and nutrients to contaminated ground water. In this approach the microorganisms are exposed to the stress conditions in the environment where they are introduced. The losses of viable microorganisms as a result of stress conditions and migration of microorganisms are the major problems with this technology. Inadequate controls over the microorganisms under specific environmental conditions limit the biological process and result in incomplete contaminant transformation.
Key requirements for success of any bioremediation process are complete detoxification of the contaminants, high removal efficiencies, and process stability and control. Known in the art is the immobilization of cells can offer stability and control for biological processes. Also known in the art is different carriers have been investigated to entrap mixed microbial cells for removal of organics from wastewater.
Cell immobilization can be defined as any technique that limits the free movement of cells. Cell mobility can be restricted by aggregating the cells or by confining them into, or attaching them to, a solid support. Historically, immobilized cells have been widely used in the wastewater treatment industry, generally through the use of undefined mixed cultures immobilized by natural flocculating tendencies or as films on solid surfaces.
Polyvinyl alcohol has proved to be a useful means of immobilization of cells. PVA-immobilization of cells is the entrapment of microorganisms within a porous polymeric matrix of polyvinyl alcohol. The porous matrix captures the microorganism cell and allows diffusion of contaminate substrates toward the cells where they can be metabolized by the cells. The matrix also permits metabolism products the pass from the entrapped microorganisms. It has been determined that entrapped microorganisms are protected against the effects of toxic chemicals compared to free cells.
Granular Activated Carbon(GAC) immobilization of cells is the attachment or adsorption of microorganisms on the surface of activated carbon. The activated carbon operates like a xe2x80x9cbuffer and depot.xe2x80x9d It protects the microorganisms and sets low quantities of toxicant for biodegradation. In contrast to a nonadsorbent material such as sand, activated carbon allows storage of substances that are difficult to biodegrade. Such storage provides a longer contact time between the microbial population and the substrates and could promote microbial acclimation and subsequent biodegradation.
Different carriers have been investigated to entrap mixed microbial cells for removal of organics from wastewater. The polymeric materials tested included cellulose triacetate (mono-carrier), polyacrylamide, K-carrageenan and a combination of cellulose triacetate and calcium alginate (bi-carrier). The mono-carrier was used to determine long term operational performance because it had better mechanical strength. The bi-carrier was more porous and more elastic than the mono-carrier. It was determined that K-carrageenan and calcium alginate were weak in mechanical strength.
Immobilized Pseudomonas sp. in alginate and polyacrylamide-hydrazide (PAAH) has been used to degrade phenol at initial concentrations of up to 2 g/L in less than two days. A sieve-like container within a fermenter held the immobilized cells in order to simulate entrapped microorganisms in a packed column. It was found that immobilization acts as a protective cover against phenol toxicity.
Biodegradation of PCP by Flavobacterium cells immobilized within polyurethane has been studied and compared PCP degradation capacities of free and immobilized cells at various initial PCP concentrations. Results showed that immobilized cells were able to degrade PCP up to a concentration of 200 mg/L, whereas free cells were unable to mineralize PCP during the four-day course of the experiment. Experiments were conducted in batch, semicontinuous batch, and continous-culture bioreactors. It was concluded that twice the amount of PCP was degraded per gram of polyurethane in the continuous-culture reactors than in the semi-continuous batch reactors. Polyurethane was determined to be an effective immobilization matrix as indicated by its protection against toxicity.
One researcher, Sofer, has studied an activated sludge of a mixed microbial population immobilized in calcium alginate gel for biodegradation of chlorophenol. Sofer was able to obtain a physically strong bead structure by optimizing the concentrations of sodium alginate and calcium chloride. The immobilized cells in Sofer""s study showed the ability to degrade chlorophenol in various concentrations (up to 100 ppm).
Various methods of producing carriers for immobilizing cells have been examined by researchers. Hashimoto and Furukawa have developed a method for immobilization of activated sludge known as the polyvinyl alcohol (PVA)-boric acid method. The preparation of this method involved mixing one portion of concentrated activated sludge (mixed microbial cell population) with one portion of an aqueous PVA solution. This mixture was dropped into a gently stirred saturated boric acid solution to form spherical beads. The beads were cured in the solution for 15-24 hours and then washed with tap water. The beads produced were used to determine removal rates of total organic carbon (TOC) and total nitrogen (T-N) from a synthetic wastewater.
The method developed by Hashimoto and Furukawa does not produce PVA beads which are long lasting and which can withstand the stress and pressures presented when the PVA beads formed by the method of Hashimoto and Furukawa are formed into a permeable barrier. The Hashimoto and Furukawa method beads fracture and compress under the pressure and generally will dissolve in less than thirty (30) days. However, the PVA-boric acid method is inexpensive compared to other methods and allows operation of an immobilized cell system at 2-3 times the contaminate loading rate of conventional systems. Since activated sludge cells become surrounded by extracellular polymer, microbial activity is not reduced during the immobilization process where the pH was 4.0 for 24 hours.
Wu and Wisecarver have prepared PVA beads using a modification of the PVA-boric acid method but added a small amount of sodium alginate to prevent or minimize the tendency for the beads to agglomerate. The viability of Pseudomonas immobilized cells was demonstrated by utilizing them in a fluidized bed bioreactor for a period of two weeks. The beads were able to withstand high shears with no sign of breakage when an 8-L fluidized bed column was sparged at an air flow rate of 1.4 L/min.
Kindzierski investigated the use of activated carbon and two other synthetic ion-exchange resins as support materials for an anaerobic phenol-degrading microorganisms. Rapid adsorption of phenol on activated carbon without bacteria occurred over the first 33 minutes. The adsorption of phenol on activated carbon with bacteria was 3.9 times smaller than on activated carbon without bacteria. Kindzierski demonstrated that activated carbon exhibited favorable qualities as a biological support for the rapid development of attached biomass. Also, a substantial decrease in the rate of phenol adsorption by activated carbon due to the colonization of the bacteria was observed.
Ehrhardt and Rehm studied the adsorption of phenol as well as Pseudomonas sp. and Candida sp. on activated carbon, and the phenol degradation by these immobilized microorganisms was compared to that of free microorganisms. They observed that one gram of activated carbon adsorbed 4xc3x9710E9 Pseudomonas cells and 3xc3x9710E8 Candida cells in about 10 hours. Results of the degradation studies showed that free cells did not tolerate more than 1.5 g/L phenol, while the immobilized microorganisms survived at temporary 2.0 hour of high phenol concentrations up to 15 g/L, and they ultimately degraded about 90% of the adsorbed phenol.
Ehrhardt and Rehm (1989) studied phenol degradation in a semi-continuous and continuous reactor by Pseudomonas putida P8 adsorbed on activated carbon. They stated that phenol introduced into the reactor was initially removed from the media by a combination of degradation and adsorption. As the biomass in the reactor increased, adsorption decreased and the degradation rate increased. They were able to show that immobilized cells on activated carbon can tolerate high concentration of phenol up to 15 g/L. They concluded that protection in the activated carbon system was afforded by adsorption of phenol onto the immobilization substrate, which reduced the aqueous concentration to which the organisms were exposed. As the phenol in solution was degraded, desorption occurred, allowing the organisms to metabolize the substrate released from the carbon.
The polymeric materials tested included cellulose triacetate (mono-carrier), polyacrylamide, K-carrageenan and a combination of cellulose triacetate and calcium alginate (bi-carrier). The mono-carrier was used to determine long term operational performance because it had better mechanical strength. The bi-carrier was more porous and more elastic than the mono-carrier. It was determined that K-carrageenan and calcium alginate were weak in mechanical strength. It is also known in the art that in contrast to a nonadsorbent material such as sand, activated carbon allows storage of substances that are difficult to biodegrade. Such storage provides a longer contact time between the microbial population and the substrates and could promote microbial acclimation and subsequent biodegradation.
The invention has advantages over the prior art in that, (1) it can reduce organic contaminants into harmless by-products by using immobilized cells; (2) it has demonstrated continuous high stability and control under many different operating conditions than previous methods; (3) it provides a very cost effective process for treatment of contaminated groundwater; (4) it has demonstrated high tolerance against environmental stresses.
The present invention solves or substantially reduces in critical importance problems in the prior art by providing a biological processes that uses immobilized cells system to treat contaminated groundwater efficiently and cost-effectively. Known in the art that immobilized cells can limit the movement of microorganisms and protect them against environmental stresses.
The present invention encompasses a method of providing a biological permeable barrier comprising a permeable barrier of encapsulated microorganisms having an affinity for a contaminate that is polluting a water supply. The invention provides decontamination of a water supply, such as a groundwater, by allowing the groundwater to flow through the biological permeable barrier comprising an encapsulated microorganism so that the microorganism selected for use in the permeable barrier can biodegrade the contaminate. During the biodegradation the microorganism converts the contaminate into a less harmful or non-harmful moiety.
This invention entails immobilizing microbial organisms which are acclimated to the target contaminants in unique immobilized systems. Immobilization is key to the ability of this process to concentrate a large active bacteria mass for treatment of contaminated water. This superior ability to concentrate active bacterial mass in the barrier offers considerable benefits to the performance of the barrier.
Entrapped or encapsulated cells are shielded from their surroundings while the target pollutants still can flow into the supports and be metabolized there. Immobilization can be a form of biocontainment since it provides a way to control the spreading of recombinant cells in the environment. Additionally, the immobilization of high cell densities in compact reactors results in enhanced biodegradation rates when compared to conventional systems. Since such a system is much less dependent on the growth rates of the microorganisms involved, short retention times can be applied and thus high removal rates attained. These characteristics make the use of the immobilized cells systems particularly attractive for the treatment of groundwater and aquifers heavily contaminated with toxic, relatively soluble pollutants.
The present invention is demonstrated by immobilizing microorganisms on example carrier materials or matrices. One example is a polyvinyl alcohol (PVA) carrier material to produce PVA-immobilized cells. A second example of a suitable carrier material is Granular Activated Carbon (GAC) which is used to provide GAC-immobilized cells. Both the PVA-immobilized cells and the GAC-immobilized cells were then formed into biological permeable barriers to clean up groundwater which had been spiked or contaminated with trichlorophenol (TCP).
The present invention thereby fulfills the following objectives: providing a biological permeable barrier media comprising a carrier material and a microorganism suitable to biotransformation of a contaminate in groundwater; providing a biological permeable barrier media which is easy to operate and lower in cost than previously used methods of treating contaminated groundwater.
In the example embodiments of the invention, described hereinafter, biodegradation of 2,4,6 trichlorophenol (TCP) is demonstrated using polyvinyl alcohol (PVA)-immobilized cells and granular activated carbon (GAC)-immobilized cells as biological permeable barrier media. A variety of conditions such as different flow rates and different contaminant influent concentrations were used to compare these to embodiments on the basis of removal efficiency, relative ease of operation, and capital cost.
The following detailed description of the present invention on two embodiments demonstrates the substantial improvement of the present invention over the prior art methods. In addition the following important operational benefits are provided by the present invention such as no precipitation of solid contaminates, no need to replace the barrier, no need to remove the barrier once it has been in operation due to collection of contaminates, no by-product contaminates are produced, complete detoxification of the contaminate can be obtained, low operation cost and maintenance cost of the barrier is presented, no sludge is produced which must be removed from the site and destroyed and no hazardous waste is produced.
The foregoing and other objects are not meant in a limiting sense, and will be readily evident upon a study of the following specification and accompanying drawings comprising a part thereof. Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of this invention.
It is a principal object of the present invention to provide unique media to immobilize microbial organisms that are acclimated to the target contaminants.
It is another object of this invention to provide an economical and environmentally safe process to biodegrdate organic compounds in contaminated groundwater. The method involves the entrapment of active microorganisms into a media that would provide controlled environment for their attachment and growth.
A further object of the invention is to provide a very stable and efficient process to treat contaminated groundwater by using immobilized cells without formation of any harmfull by-products.
These and other objects of the invention wil become apparent as a detailed description of representative embodiments proceeds.